Also, the failed Invasion of Afghanistan signaled to Soviet Satellite states that the Red Army weren't as invisible as previously believed.
The economy of the Soviet Union was also weaker than that of the West, which they rivaled.
The fall of the Soviet Union, much like the fall of the Roman empire, wasn't down to just one or two or three factors, but was a culmination of factors.
The Soviet Beast became too big and collapsed on itself.
I wrote about the book Midnight in Chernobyl by Adam Higginbotham [1] on my blog [2] as it's one of the best books I've read. I highly recommend it.
I'm just going to copy and paste from my blog -
The Chernobyl explosion wasn’t the result of a single mistake, but the inevitable outcome of a flawed reactor design combined with secrecy, bureaucracy, and human error. On April 26, 1986, at exactly 1:23:40 a.m., the reactor exploded with the force of 40–70 tons of TNT, launching its 1,000-ton lid through the roof like a champagne cork.
Firefighters arrived within minutes, confronting intense fires with bare hands and leather helmets, unaware that radiation levels were measured in thousands of roentgens per hour; many died within weeks. Despite knowing by noon that the reactor core was exposed and radioactive smoke was drifting across Europe, Soviet authorities kept Pripyat open for 36 more hours, letting children play in radioactive fallout.
Three men volunteered to wade through radioactive water beneath the melting reactor to open valves and prevent an even greater steam explosion. All three survived.
The cleanup required 600,000 “liquidators”, mostly conscripted soldiers and reservists, who shoveled radioactive debris with bare hands and were exposed to lifetime doses in minutes. The initial test that triggered the explosion had been delayed ten hours to keep Kyiv powered for the evening, leaving exhausted operators to run the reactor on a skeleton crew they barely understood.
Watch the HBO miniseries Chernobyl. It's one of the greatest things I've ever watched.